Syngas and its components, hydrogen and carbon monoxide, are conventionally produced by the high temperature partial oxidation of hydrocarbons with controlled amounts of air or oxygen. Although air is less expensive and more convenient to use in partial oxidation reactions, it is less attractive than oxygen for such reactions because the large quantities of nitrogen that are produced when air is used as the oxidant must be subsequently separated from the product gas prior to its use. In addition, the heat energy carried by nitrogen must be recovered before nitrogen leaves the process. The cost of separation, purification and heat exchange equipment for product gas purification and recovery of heat energy from the nitrogen adds considerably to the cost of syngas production using air.
Although oxygen is more desirable than air as an oxidant for partial oxidation reactions, its use is not without disadvantage, in that oxygen must be imported into the system, or it must be generated on site, for example, by means of a cryogenic air separation plant or an adsorption system. In either alternative, using oxygen as the oxidant likewise adds considerably to the cost of the process.
More economical methods of on site production of oxygen for applications such as hydrocarbon partial oxidation reactions are continuously sought. U.S. Pat. No. 5,714,091 discloses an oxygen-based hydrocarbon partial oxidation process in which the oxygen is produced on site by subjecting air to membrane separation using a membrane constructed of perovskite-type ceramic material. Oxygen, which is permeable to the membrane, passes through the membrane and is made to react with hydrocarbons on the downstream side of the membrane unit. The disadvantages of this method of oxygen production are the high cost of production of the membrane and the difficulty of producing membrane structures that are leak-proof.
The partial oxidation of hydrocarbons with oxygen retained in a ceramic-based oxygen-selective mixed conducting substances, such as perovskite-type ceramics, is disclosed in copending U.S. patent application Ser. No. 09/175,175, filed Oct. 20, 1998, the disclosure of which is incorporated herein by reference. The process disclosed therein is a cyclic adsorption process which produces syngas and nitrogen-rich product gases on a semi-continuous basis.
The present invention provides a partial oxidation process that likewise uses a ceramic oxygen-selective mixed conductor, but which has the advantage over the above process of producing hydrogen, carbon monoxide and nitrogen-enriched product gas on a continuous basis.